Since Fujishima & Honda reported the photosensitized decomposition of water into H2 and O2 (Nature (1972) 238, 37-38), much research has been undertaken on the application of photocatalytic semiconducting material in functions as far-ranging as air de-odorization, air purification, anti-fogging materials, water treatment, anti-bacterial materials, self-cleaning materials, and biomedical materials. The basic principle of the progress of a photocatalytic process can be described as follows. Upon exposing semiconducting materials to matching irradiation light (such as UV, visible or solar light), excitation by photons from the source causes electrons in the valence band of the exposed materials to be promoted into the conduction band over the band-gap. As a result electron (in the conduction band)-hole (in the valence band) pairs are formed. After migration to the solid's surface, these electron-hole pairs may undergo electron-transfer processes with adsorbates of suitable redox potentials to decompose organic compounds including the deactivation of bacteria or degradation of organic dyes (FIG. 1).
The conventional photocatalyst TiO2 has a 3.2 eV wide bandgap. Thus it can be excited under wavelengths of λ<388 nm, i.e. under UV irradiation. These wavelengths however constitute only ≦4% of the solar energy (FIG. 2). Hence, its poor photo-catalytic efficiency under visible light has limited its use in indoor and outdoor applications involving the decomposition of organic compounds. There is therefore a need for photocatalytic matter that is capable of utilizing the visible light spectrum, i.e. wavelengths in the range from about 400-700 nm that constitute 43% of the solar energy.
For this purpose many researchers have paid particular attention to search or synthesize visible-light-driven photo-catalysts (the material bandgaps between 3.1-1.8 eV for absorbing visible light (Shu, Y., et al., J. Mater Chem. (2003) 13, 2996-3001). To date, the development of photo-catalytic materials under visible irradiation can be classified into 3 basic research directions:
Firstly, the conventional photocatalyst TiO2 series, doped dopants or coupled with narrower band-gap semiconductors, can absorb a small part of the visible light (Kumar, S., et al., Applied Catalysis B: Environmental (2005) 57, 93-107; Bessekhouad, Y., et al., Catalysis Today (2005) 101, 315-321). TiO2 doped with Cr, V or N produces some red shift in the energy band resulting in the absorbance of a small part of the light in the visible range. TiO2 coupled with the narrower band-gap semiconductor materials (as sensitizers), such as the Cu2O/TiO2, Bi2O3/TiO2, and ZnMn2O4/TiO2 hetero-junctions, can absorb some single visible light frequency.
Secondly, narrow bandgap materials can directly absorb some single visible light frequency, including some toxic binary-compounds (e.g. CdS, ZnS, CdSe, ZnSe, CdTe), and poly-compound MTaO2N (Ta5+-based (oxy) nitrides), Bi2MNbO7, BiMO4, InMO4, (M represents a metal) etc. (Peral, J., et al., J. Chem. Technol. Biotechnol. (1997) 70, 117-140; Yamasita D., et al., Solid State Ionics (2004) 172, 591-595).
Thirdly, doping dopants in poly-compound with wide band-gap (above 3.1 eV) can absorb a small part of the visible light due to some red shift in the energy band (Yamasita D., et al., 2004, supra). For example, SrTiO3 (3.2 eV band-gap) doped La, N, F, Cr or Ta can produce a red shift that allows part visible light absorption (Wang, J., Journal of the European Ceramic Society (2005) 25, 3207-3212).
All these reported photo-catalysts can only utilize a part of the visible-light range due to either a limited red shift of energy band or single visible light frequency dependence for the hetero junction materials. Hence they have very poor photo-catalytic efficiency under visible light and can only be used for limited applications such as deodorizers, anti-fogging materials and self-cleaning materials. There thus remains the need for a photocatalyst that can absorb a wider range of the visible light spectrum, ideally the whole visible light range, i.e. the wavelength range from about 400 to about 700 nm. It is furthermore desirable for anti-bacterial applications under visible light, that the photo-catalytic reaction rate for the respective materials be faster than the growth rate of the bacteria, in order for the material to prevent bacterial growth.
Various embodiments provide a method of providing photocatalytic activity with properties that overcome at least some of the above discussed disadvantages.